Phenol formaldehyde resins are synthesized by condensation polymerization of phenols and aldehydes, and most importantly by the condensation polymerization of phenol and formaldehyde. Phenol formaldehyde resins feature advantageously ease of access to raw materials, low water absorbency and excellent workability so that, when cured, phenol formaldehyde resins meet practicability needs and thus are in wide use to serve consumptive, electronic, military and architectural purposes. Recent years see the emergence of benzoxazine resin, a member of phenol formaldehyde resins. Benzoxazine resin is characterized in that, when heated, its monomers undergo a ring-opening reaction to cure, and the curing process occurs without producing water or being accompanied by any other side reaction; hence, benzoxazine resin exhibits better workability than the other phenol formaldehyde resins.
In 2005, Takeichi synthesized from aromatic diamine, bisphenol A and paraformaldehyde a polymeric benzoxazine resin (also known as polybenzoxazine precursor) with a weight-average molecular weight Mw of 6000-8900 g mol−1 and a number-average molecular weight Mn of 2200-2600 g mol−1. When film-coated and cured at high temperature, the polymeric benzoxazine resin surpasses non-polymeric benzoxazine in mechanical strength and toughness as well as has a glass transition temperature Tg of 238-260° C. because its main chain displays a high crosslinking density. Hence, the brittleness of polybenzoxazine polymers is alleviated.
In 2000, Oihara Tamotsu synthesized benzoxazine from three different diphenol monomers by a conventional one-step technique and measured its mechanical properties and dielectric constant, thereby drawing conclusions as follows: the benzoxazine synthesized from conventional aromatic diphenol monomers has a dielectric constant Dk of 3.49; the benzoxazine synthesized from aliphatic bisphenol has a dielectric constant Dk of 3.22; and, with adamantane having a rigid structure, introduction of an adamantane structure-containing bisphenol increases the glass transition temperature of its cured substance and reduces its dielectric constant Dk to 3.06, thereby indicating that the introduction of an adamantane structure enhances the mechanical properties and glass transition temperature Tg of its cured substance and reduces its dielectric constant.